How do you make the change from 1 player 3 buttons, to 2 player 6 buttons (per player)?

If you can't already tell, I'm newer to this hobby. As I'm looking for my first candy cab, I keep seeing more and more cabinets without the right amount of buttons and players that I want. So, assuming that the one I finally buy has the wrong amount of buttons, how would I go about fixing that problem? I would need to buy a new control panel, duh, but what do I need to do with the wiring and what would I need to buy to add these things in. I really haven't looked into wiring buttons and joysticks at all, so I don't know how these things work.

Assuming that the cabinet you end up getting comes with all the wiring intact, it should be only a matter of connecting the joysticks and buttons to the quick disconnects under the cp, the jamma standard is wired for 2 players and 3 buttons each + start, for the 3 extra buttons you will need a "kick harness" which is basically a pair of 3 extra buttons for those pcbs that use them (mostly fighting games), but be aware that not all cabinets have it stock, specially if the cab doesn't come with a 2L12B panel.

If it doesn't come with all of the wiring intact, what is the wiring called under there so I could buy some for more buttons and joysticks?

You can either get a generic jamma loom or try to source replacements of the original wiring (or fix it yourself), the later being easier in case you are getting a Sega cab vs other brands.

Original or up to spec would be best since most candy cabs wiring use AMP or Mate N Lok connectors which will make your life easier when working inside the cab, rotating the screen, swapping boards etc...and it will also make the innards of the cab to look tidier and cleaner.